In a hot-dip continuous galvanizing line or the like, a galvanized steel strip is rolled by work rolls and is subjected to skin pass mill rolling to provide an appropriate steel-roughness to a galvanized surface of the steel strip. In such a skin pass mill, when the galvanized steel strip is rolled, a part of the galvanized layer is peeled off to generate sticking substances (e.g., zinc powders). The sticking substances are adhered on surfaces of the work rolls on which convex and concave portions are formed to provide steel-roughness. The adhered sticking substances are accumulated in the concave portions to reduce roughness of the surfaces of the work rolls, resulting in clogging of the work rolls. Because of the clogging of the work rolls, necessary steel-roughness is not given to a hot-dip galvanized steel strip after the skin pass mill rolling, so that press moldability and image clarity requisite when the hot-dip galvanized steel strip is used as an automobile member and the like cannot be secured. For this reason, there has been used a typical cleaning method of removing sticking substances (e.g., zinc powders) by scratching a surface of a work roll with a contact-type cleaning brush or the like. However, in such a case, the surface of the work roll is scraped to reduce surface roughness.
Accordingly, there has been proposed a cleaning method of removing sticking substances with surface roughness of the work roll kept at a predetermined condition (see, for instance, Patent Literatures 1 to 3).
A skin pass mill disclosed in Patent Literatures 1 and 2 includes opposing cleaning nozzles spaced at a predetermined distance from surfaces of work rolls. High pressure fluid is squirted over the surfaces of the work rolls through the cleaning nozzles and sticking substances are blown out and removed by the pressure of the high pressure fluid. Examples of the high pressure fluid used in such a cleaning device include rolling oil, air, oil mist and hot water.
A skin pass mill disclosed in Patent Literature 3 coats rust preventive oil on a surface of a to-be-rolled galvanized steel strip before rolling, thereby preventing adhesion of sticking substances on a surface of a work roll and keeping surface roughness of the work roll.
On the other hand, unlike the method of Patent Literatures 1 and 2 in which only high pressure fluid is squirted through the cleaning nozzles, there has been proposed a cleaning method using cavitation jet in air (see, for instance, Patent Literature 4).
According to the cleaning method disclosed in Patent Literature 4, high pressure water is squirted from the center of a nozzle while low pressure water is squirted from the surroundings around the center, so that cavitation phenomenon is generated at a position where high pressure water and low pressure water collide, thereby cleaning an object. The cleaning method using such cavitation jet in air can provide higher cleaning effects than the cleaning methods disclosed in Patent Literatures 1 and 2 using the high pressure fluid, even when the pressure of the high pressure water is not set at such a high pressure.